Clinical samples from patients with acute or chronic non-A, B, C, D, E hepatitis in the United States are being studied for biological, serological or molecular evidence of transmissible agents. Patients with fulminant non-A to E hepatitis remain a diagnostic enigma and may be infected with one or more previously unrecognized viruses. We are attempting to discover the etiology of this disease. Evidence for the existence of an additional water-borne hepatitis virus has come from our seroepidemiologic studies in India, Egypt and Saudi Arabia. We are attempting to transmit an agent from clinical specimens of such patients. Some years ago, a hepatitis virus was reported to be transmissible from a non-A, non-B hepatitis patient to marmoset (tamarin) monkeys. The agent, called the GB agent, could be serially transmitted in marmosets and was partially characterized. The GB agent was cloned and sequenced and shown to be two separate viruses (GBV-A and GBV-B), each distantly related to hepatitis C virus as well as to a previously unrecognized human virus (GBV-C). The GB agents and the newly discovered human virus have been studied in primates and in vitro. An infectious cDNA of GBV-B was constructed. The newly recognized human virus (GBV-C) was transmitted to chimpanzees and the resulting infection characterized. In addition, several new viruses, related to GBV-A, were discovered in New World monkeys. Hepatitis E virus may be emerging as a greater public health problem than previously thought. We are studying its epidemiology in developing and industrialized countries worldwide. Serologic evidence of infection of swine with hepatitis E virus (HEV) was obtained. A new and unique HEV strain was recovered from infected swine and characterized. It was shown to have a worldwide distribution. Seroepidemiological studies of swine handlers and matched blood donors have shown an excess of antibody to HEV in swine handlers, suggesting that the virus may be zoonotically spread. Similar serologic evidence for infection of wild rats with HEV has also been obtained and the infecting agent is being sought. To date we have successfully transmitted the agent from rats trapped in Los Angeles to laboratory rats of the same species (Rattus norvegicus). However, transmission has been difficult, suggesting that the virus replicates at low titer. We have prepared a relatively high-titered pool of the virus in SCID rats. Studies to determine if rat HEV is linked to human infection are in progress. Modern techniques of molecular biology have been used to discover new viruses in recent years. These are now being applied to prospectively collected sera from patients with transfusion-associated hepatitis in a search for new hepatitis viruses that may cause up to 15% of such hepatitis in the US. In an attempt to increase the sensitivity of specific virus discovery technologies (i.e., those capable of amplifying unique sequences without regard to the sequence), we have introduced modifications to existing techniques that have improved the sensitivity of the assay. Using the new technology, we have discovered two previously unrecognized bovine parvoviruses that contaminate most lots of bovine serum. This finding has implications for the safety of biologicals that are prepared with bovine serum. Kawasaki Disease is a life-threatening illness of young children. It has the epidemiologic characteristics of an infectious disease. The HVS is attempting to transmit a putative agent from acute phase clinical samples of children with Kawasaki Disease to nonhuman primates.Another newly recognized pathogen is the etiologic agent of Severe Acute Respiratory Syndrome (SARS). We are studying the natural history of this virus in relevant animal model systems.